In conventional heat pipes a refrigerant is cooled in a condenser so as to form a condensate. The condensate is transported to an evaporator at the other end of the pipe by capillary action in a wick, the wick generally extending from one end of the pipe, in the condenser to the other end of the pipe, in the evaporator. When the condensate moves into the evaporator, it is vaporized by the application of heat to the evaporator wall. The vaporized refrigerant removes heat from the evaporator wall and stores it as latent heat of vaporization. The vapor moves toward the condenser because of a slight pressure difference between the evaporator and the condenser. In the condenser, the vapor is cooled and the condensate is formed. As it is condensed the refrigerant gives up its latent heat of vaporization to the condenser wall, a cooling device being adapted to carry the latent heat away. Thus, in the process the refrigerant acquires latent heat of vaporization in the evaporator and loses it in the condenser. Heat pipes have thus been used as a means of removing heat from one area and disposing of it in another.
The action of a conventional heat pipe stops if one of several heat pipe limits are reached. The most significant of these is the lack of adequate capillary pumping action to supply the evaporator with fluid. For wick materials such as screen or porous metals, this limit can be expressed in equation form as:
2.sigma./.sup.r min=.sup..DELTA..rho. static+.sup..DELTA..rho. flow, where.sigma.= surface tension, .sup.r min=minimum meniscus radius allowable, .sup..DELTA..rho. static= hydrostatic pressure due to the evaporator elevation being greater than the condenser elevation, and .sup..DELTA..rho. flow=flow pressure drop.
For typical heat pipe working fluids such as ammonia, where the wick is 400 mesh wire cloth, the maximum value of 2.sigma./.sup.r min is 18.6 cm.
The first of the two pressure drops, .sup..DELTA..rho. static, exists only in gravitation or acceleration environments and does not, therefore, affect spacecraft heat pipes in flight. It does occur, however, in ground testing and therefore must be considered even in spacecraft heat pipes.
The second pressure drop term, .sup..DELTA..rho. flow affects all heat pipes and arises from viscous drag on the moving fluid. In many designs, this restriction has been decreased by employing arteries consisting of tubes formed by fine mesh wire cloth, sealed at the ends, and in liquid communication with both the evaporator and condenser. When primed, or filled with fluid, the arteries take on the capillary pumping capability of the pores of the artery wall, but have a much bigger flow channel cross section and therefore, less pressure drop, than a simple wick consisting of stacked layers of wire cloth. The arteries thus permit an increased flow of fluid and higher heat transport rates, with increased artery diameter required as the heat pipe is made longer. A limit exists, however, on the diameter, and therefore, the capicity of an artery. When testing in one g, this limit relates to the maximum diameter which can be primed because artery priming requires that the fluid "climb" or "rise" to the top of the artery.
The use of an artery, although permitting an increase in heat transfer rate, can also create another failure mode, namely, arterial vapor bubble entrapment. When this occurs, the artery deprimes because the radius of curvature, instead of being that associated with the screen pores, is now the radius of the artery. Bubble formation in arteries occurs as the result of vibration, shock or rapid temperature fluctuation.
In general the capillary pumping limit places operating constraints on heat pipe operation in that the vaporization rate in the evaporator cannot exceed the capillary pumping rate and the height of the evaporator above the condenser cannot exceed the capillary wicking height. Capillary action pumping also limits the heat flux that can be applied to the evaporator. Because vapor bubbles in the wicking materials can effect "dry out" and stop the capillary pumping action, the heat flux must be kept below that of the nucleate boiling regime. This is the "nucleate boiling limit".
Heat pipes with wicking materials in the vapor flow passages are also subject to the "entrainment limit" which arises at vapor flows high enough to entrain liquid droplets from the wick.